Many manually operated laboratory filters exist today which permit the removal of particles or solids from fluids producing a filtrate which may then be analyzed or put to other predetermined uses. For the most part, the degree of filtration is a function of the filter employed. It is possible, with a filter which can be held in one hand, to generate up to 100 lbs, per square inch to force unfiltered liquid through extremely fine filter material separating out unwanted solids.
Typically a laboratory type, manually operated filter is one in which a hollow piston slides within a cylinder, the cylinder being open at one end and closed at the other. The piston likewise, is open at one end and partially closed at the other, the partially closed end mounting appropriate filter material. Sealing means in the form of a gasket are employed between the exterior piston wall and the interior cylinder wall to create pressure within the cylinder to force liquid through the filter.
Unfiltered liquid is first poured into the cylinder and the piston is then forced into the cylinder with the seal causing the space between the cylinder and piston walls to be pressurized. This initially forces air out of the cylinder and through the filter into the hollow, open piston. When the filter, which is located at the partially closed end of the piston, engages the unfiltered fluid, the then compressed air in the chamber forces the fluid up into the hollow piston from which the filtrate may then be withdrawn and processed.
Filtering devices of this type are shown in U.S. Pat. Nos. 4,454,231, 4,456,690, 4,510,058 and 4,587,221. They have proven to be successful in carrying out the transfer of one or more components from one liquid phase to another liquid phase, the two phases being substantially immiscible.
An earlier United States Patent to Shapiro, U.S. Pat. No. 3,512,940, discloses a pressure or vacuum filter which is used with a common laboratory test tube in the manner described above. The filter consists of a hollow plunger which acts as a piston. The plunger has a porous bottom portion which is a filter. When the piston is inserted into a test tube which contains an unfiltered fluid and forced downly thereof, the filtrate is forced upwardly into the hollow plunger from which it is subsequently removed for testing.
Conversely, the procedure may be reversed. First the plunger is forced all the way into the test tube and unfiltered liquid is placed in the hollow plunger. The plunger is withdrawn from the test tube, thereby creating a vacuum which will draw the fluid through the filter. The plunger may then be removed and the filtrate removed from the test tube. This device has proven to be a relative simple and quick means for extracting filtrate from an unfiltered fluid.
One limitation of the Shapiro device is the fact that test tubes, being round at the bottom and the plunger being flat at the bottom, always left unfiltered fluid in the test tube. In one version of the Shapiro device a substantial amount of fluid remains in the test tube because the downward movement of the plunger into the test tube is intentionally stopped at a predetermined point to prevent deformation of the filter which was in the form of a cup-like member projecting from the bottom of the plunger.
In modern medicine, more and more tests are performed on biological samples taken directly from humans. One process is to pick up a biological sample by means of a cotton swab or the like, extract the sample from the swab and test it for whatever organisms may be present. The cotton swab is immersed in an extracting fluid called an extraction buffer. The sample could include, for example, a mixture of saliva, blood, mucus, cells, bacteria, viruses, and the like. Since the sample is generally small in volume, only small amounts of extraction buffer can be employed, so as not to dilute the sample excessively.
The sample is then filtered to remove the solids, such as blood clots, mucus and cells. The filtrate, comprising the extraction buffer, viruses, bacteria and the like, is then removed for analysis. Hence, it is important to have all of the filtrate pass through the filter so that none will be wasted and remain in the filtering device.
Because of the construction of the Shapiro device, it is not capable of extracting substantially all of the filtrate. This is particularly so because of the use of round or arcuate bottomed test tubes, and because of the desire not to crush the cup-like filter which projects from the bottom of the plunger.
As a result, an object of the present invention is to provide a filtering device of the cylinder/piston type, which is capable of recovering substantially all of the filtrate from an unfiltered fluid containing a sample.